Fix: prevent useless pathfinder run for blocked vehicles #8568
Motivation / Problem
As found in #7670, a road vehicle blocked by another vehicle may enter in an "loop" of intensive pathfinder calls.
This solution is similar to the one in #7822, but a lot less invasive and does not require a savegame bump.
I only tested using the save from #7670 (comment) but I don't see how it could cause issues.
Checklist for review
Some things are not automated, and forgotten often. This list is a reminder for the reviewers.